This invention relates to systems for measuring the water vapor content of a gas mixture, and more particularly, to systems for suppressing the water vapor cross sensitivity in a non-dispersive infrared absorption gas analyzer.
Infrared absorption gas analyzers operate utilizing a physical property of gases in which the different atomic structures of different gases absorb corresponding spectral regions of infrared radiation. Such absorption of infrared energy by gas contained within a closed volume causes a measureable increase in its temperature and pressure. The concentrations of predetermined gaseous components within a gas mixture can be determined from temperature and pressure measurements.
A typical prior art system for obtaining the desired temperature and pressure measurements utilizes a source of infrared radiation which transmits electromagnetic energy in the infrared range through a measurement cell which contains a sample of the gas mixture which is to be examined, and a receiver chamber which contains a sample of the gas component, either in pure form or mixed with other gases, of the type which is desired to be measured. The measurement cell and the receiver chamber, which are each disposed in the path of the infrared energy, contain windows of a material which is transparent in the infrared spectral region, typically having wavelengths in the range between 2 and 10 microns, so as to permit the energy to propagate through the various gases.
It is a problem in the art that the gas mixture contained in the measurement cell contains gaseous components which cause "cross sensitivities". Such cross sensitivities may be caused by water vapor contained in the measurement gas, which interferes with the measurement of other gaseous components. It is desirable to eliminate the interfering component by interposing a filter cell, which is commonly filled with a filter gas corresponding to the interfering component, and which is disposed in the radiation path so as to absorb the particular interfering spectral components from the radiation. Alternatively, in the case of water vapor, it may be desirable to measure the water vapor component by filling the receiver chamber with water vapor. Thus, infrared radiation which passes through the gaseous mixture in the measurement cell would also propagate through the receiver chamber, affect the thermodynamic state of the water vapor in the receiver chamber, and such changes could be converted to electrical signals for measuring the water vapor content of the gaseous mixture in the measurement cell. The problem arises that it is difficult to contain water vapor in either a receiver chamber or in a filter cell because of its instability at ambient temperatures. Water vapor has a relatively low dew point and will tend to absorb the infrared radiation as a function of its temperature.
It is therefore an object of this invention to provide a gas which absorbs infrared energy in wavelength bands which correspond to those of water vapor, and which is chemically stable at ambient temperatures.
It is a further object of this invention to provide a stable gas which has an infrared absorption characteristic similar to that of water vapor, and which is not in general use as a gas to be measured.
It is another object of this invention to provide a gas which can be mixed with other non-absorbing filter gases such as nitrogen or argon, or with another absorbing gas, such as carbon dioxide.